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Activation energy


Changes in matter ] Importance of chemical reactions ] Chemical Reactions ] On-line practice ]


The change in energy can be plotted against the progress of a reaction, as the reactants turn into products.

Energy Diagram for an Exothermic Reaction


Going from reactants to the top of the curve, you are going up the energy scale.
Energy (heat) is being put in to break bonds in the reactants.

At the top of the curve, the bonds in the reactants have been broken.
The amount of energy put in to break these bonds is called the activation energy.

The activation energy is the minimum amount of energy needed for the reaction to occur.
A catalyst may work by lowering the activation energy for a reaction.


Going from the top of the curve to the products, you are going down the energy scale.
Energy (heat) is given out as bonds form in the products.

The reactants are higher up the energy scale than are the products.
The amount of energy (heat) you need to put in (the activation energy) is less than the amount of energy (heat) you get out.
This is a typical exothermic reaction.

The difference in energy levels between the reactants and the products is given the symbol H (the enthalpy change or the heat of reaction).
This is the amount of heat given out (or taken in) during the reaction.
For an
exothermic reaction, H is negative.
For an
endothermic reaction, H is positive.


Exothermic and endothermic changes ] Energy level diagrams ] [ Activation energy ] Rates of reaction ] Factors affecting rate of reaction ]